It's slightly more than one rule.
Shuffle the tiles and place them in a stack face down.
Take the top tile and place it face up on the table.
Each turn, the active player takes the top tile and
adds it to the tableau. It must align to the small
squares on the tiles and must touch but not overlap
some tile on the table. You score one point for each
small square in coloured regions enlarged by your
new tile; these regions must be orthogonally
contiguous and must be on both old and new tiles to
count.
Play until the stack is exhausted and the winner is
the one with the most points.
Skill is greater if you deal out the tiles first
but thinking time is larger.
Steve